Pre-insulated pipeline is fabricated by putting together lengths of manufactured pipe. The pipe lengths are manufactured in various diameters and lengths, and are typically steel pipe, optionally epoxy coated, covered with a layer of foam insulation, then covered with a polymeric outer surface jacket. The metal pipe is exposed at the two ends of the pipe length, along a defined length of pipe. One exposed length of the metal pipe is girth welded to the end of the pipeline, forming a cut-back region of bare steel pipe, having at its centre a girth weld, and surrounded on both sides by the foam insulation and polymeric outer surface jacket. Ideally, a pipeline should have continuous insulation and outer polymer jacket layers, so the cut-back region is filled using a variety of different methods and means. Typically, a casing of polymer is attached to the outer polymer jacket layer on both ends of the cut-back region, and injection filled with foam insulation.
Attachment of the polymer casing to the outer polymer jacket is typically done in one of three methods: electrofusion, induction fusion, or adhesively bonding of the inner layer of the polymer casing to the outer layer of the polymer jacket.
Certain methods of electrofusing a casing to a field joint are well known in the art. For example, U.S. Pat. No. 4,629,216 published Dec. 16, 1986 and incorporated herein by reference, describes non-shrink plastic casings and employs electric heating elements to form a bond between the plastic casing and the plastic jacket of preinsulated pipes. U.S. Pat. No. 4,866,252, published Sep. 12, 1989 and incorporated herein by reference, discloses a connection between preinsulated pipes having a casing and sleeve articles, one disposed over each end of the casing where it overlaps the jacket of the preinsulated pipe. The articles have a bonding material that will form a fusion bond to the jacket, an outer heat shrink layer and a built in electrical heating element in contact with the heat shrink layer for heating and shrinking the outer layer. Since the heat flux from the built in heating element is relatively small, the heat shrink layers are made thin to permit heat penetration and shrinking of the heat shrink layer, and according to the patent the thickness of the article before heat shrinking may be up to 6 mm. The article is less useful where thicker casing members are desired for use with large diameter preinsulated pipes. Use of electric heating elements for binding casings is also described in the Mounting Instructions for the Electric Welder for BelmaFlex Welding Joints (BelmaFlex, Farsø, Denmark). The BelmaFlex casing system comprises a welding band which is a heating element, fitted to the outer surface jacket of the pipe on each end of the pipe joint. A casing is then slid over the pipe joint, and the ends of the casing are heat shrunk to the outer surface jackets on each end of the pipe joint. Once the casing has cooled, buckles are placed around the heat shrunk sections, and current is applied to the welding band, through electrical connections connected to the welding band and extending beyond the casing, between the casing and the outer surface jacket. The application of electric current causes the welding band to heat, which melts and fuses the casing to the outer surface jacket. The casing is then drilled, foam insulation is injected into the drill holes to fill the gap between the casing and the exposed pipe joint, and the drill holes are capped.
Some of the present inventors have also disclosed various methods for electrofusing of a casing member to a cut-back region of a preinsulated pipeline, and casing members for use in those methods, for example, in PCT publication WO 2011/143733, incorporated herein by reference. Generally, the casing members have a first, cross-linked, heat shrinkable outer layer, and a second, inner, non-crosslinked (or less cross-linked) layer. The casing members have an electrically heatable member spaced inwardly from the first layer, for fusion bonding the second layer to the tubular sections.
In use, when the first layer of the casing member is heat shrunk, in one preferred form it applies a hoop stress to the second layer. When the second layer is heated with the electrically heatable members, in a preferred form, a fusion bond is created between the material of the second layer and of the outer surface of the tubular section. A fusion bond is created when two compatible plastics materials melt and fuse together under fusion or welding conditions. A fusion bond results in a continuously homogeneously weld portion. The above-mentioned hoop stress facilitates formation of the fusion bond.
The WO 2011/143733 publication further describes the use of an electrically heatable member as a particularly effective and convenient way of heating the second layer and the tubular section surface substrate to create the fusion bond. The electrically heatable member, may for example, be in the form of electrical resistance heating elements or electrically inductively heatable elements which may be disposed inwardly from the second layer, or may be disposed integrally within the second layer.
Generally, a proper electrofusion of two surfaces has excellent mechanical properties and is difficult to unattach. However, some prior art electrofusion methods have been criticized as sub-optimal for water penetration of the seal.
A known alternative method to electrofusion is to bond the outer polymer surface layer of the pipe to the casing using an adhesive. Typically, the adhesive is in the form of an inner layer of the casing. In some methods, the adhesive is a liquid adhesive which is applied to the outer polymer surface layer or to the inside of the casing (or to both). In most instances the adhesive must be heated to bond to at least one of the two surfaces. While adhesive-based methods are very well known and used with much success, some adhesive-based methods are criticized for less than optimal mechanical properties, and are known to detach with time and/or application of external environmental factors.